This invention relates to amplifiers generally and particularly to an apparatus for providing amplification of a video signal for driving the cathode electrode of a kinescope.
In television apparatus employing direct view or projection kinescopes as display devices, it is desirable that the amplifier driving the kinescope cathode provide a relatively high voltage drive signal having a wide bandwidth and a high slew rate. Typically, drive voltages may be on the order of 200 volts or more and bandwidths may be substantially higher than conventional television standards in certain applications, for example, where both conventional TV viewing and data display are desired. Even higher bandwidths may be required in video applications requiring scanning at two times of more of the standard TV line rate.
To facilitate high voltage operation it is common to employ a cascode configuration of a common emitter input stage driving a common base output stage. Such a configuration requires only one high voltage transistor (the output stage) and since it is connected in a common base configuration the Miller effect is suppressed and very wide bandwidth operation is thus possible. In practice, the actual bandwidth and slew rate which may be achieved in a cascode amplifier depends, to a great extent, on the effective load capacitance presented to the output stage and the available output current.
In general, one may either increase the amplifier operating current or decrease the effective load capacitance to maximize the bandwidth and slew rate of the amplifier. However, since increasing the current necessarily implies increasing the amplifier power dissipation, it is preferable to take steps to reduce the effective load capacitance for improved performance rather than resort to increases in operating power.
In kinescope driver applications, the xe2x80x9ceffectivexe2x80x9d load capacitance presented to the amplifier is principally that of the kinescope cathode and stray capacitances associated with the socket, spark gaps, wiring and the like. An effective approach to reducing the effective capacitance loading is to couple the amplifier to the cathode by means of a push-pull complementary emitter follower amplifier. Such an amplifier effectively xe2x80x9cisolatesxe2x80x9d the load capacitance approximately in proportion to the reciprocal of the transistor current gain (xe2x80x9cbetaxe2x80x9d). The additional current provided by the follower amplifier provides faster charging and discharging of the load capacitance and thus enhances slew rate and bandwidth. To avoid substantially increasing the quiescent power dissipation, it is customary practice to operate the follower amplifier in a xe2x80x9cclass-Bxe2x80x9d mode in which the push-pull transistors are biased to avoid simultaneous conduction.
An example of a kinescope driver amplifier in which the load of a cascode amplifier is coupled to the cathode of a kinescope via a push-pull complementary emitter follower amplifier for load capacitance reduction is described, for example, by John H. Furrey, in U.S. Pat. No. 4,860,107 entitled VIDEO DISPLAY DRIVER APPARATUS which issued Aug. 22, 1989. Advantageously, the use of a complementary emitter follower driver in the Furrey apparatus significantly reduces the effective load capacitance of the display (kinescope load and stray capacitances) thereby improving positive and negative video signal transient response.
It has been recognized by White et al. in U.S. Pat. No. 5,680,173 entitled KINESCOPE DRIVER APPARATUS, which issued Oct. 21, 1997, that significant further improvements may be made in kinescope driver amplifiers of a type having a complementary emitter follower output coupling stage. Specifically, in the White et al. apparatus a push-pull complementary emitter follower amplifier is coupled between the output of a high voltage driver amplifier and the cathode of a kinescope for reducing the effective capacitance presented to the driver amplifier that is attributable to the kinescope cathode, socket, spark gaps and related stray capacitance. A secondary undesired capacitance loading of the amplifier attributable to the collector-to-base capacitances of the follower amplifier is effectively reduced by regulating the collector-to-emitter voltages of the push-pull follower output transistors at respective substantially constant values thereby improving parameters such as the slew rate and bandwidth of the overall video display system.
In the prior art discussed above, a desirable reduction in load capacitance has been achieved by using emitter follower load isolation (Furrey) and a further reduction in load capacitance was achieved by regulating the collector-emitter voltages of the follower transistors (White et al.).
It is herein recognized that a need exists for further improvements in kinescope driver amplifiers of a type employing positive feedback for emitter follower capacitance reduction in applications where it is also desired to provide accurate sensing of the kinescope cathode current for purposes such as automatic kinescope bias (hereafter xe2x80x9cAKBxe2x80x9d) control. The present invention is directed, in a first respect, to meeting that need.
The present invention relates to kinescope driver apparatus of a type comprising a complementary emitter-follower amplifier having an input coupled to a video amplifier for receiving a video signal and having an output coupled to a cathode of a kinescope. A feedback circuit is provided for applying respective positive feedback voltages to respective collectors of first and second output transistors of the complementary emitter follower for maintaining substantially constant collector-to-emitter voltages for the output transistors and an AKB current sensor is connected in a collector circuit of the second output transistor for sensing cathode current of the kinescope.
In accordance with the invention, the output of the complementary emitter follower is coupled via a threshold conduction switch means to the emitter of the first output transistor and is coupled via resistive means to the emitter of the second output transistor.
In accordance with a further feature of the invention, the feedback circuit has an input connected to a junction of the threshold conduction switch means and the emitter of the first output transistor.
In accordance with another feature of the invention, a capacitor is coupled in parallel with the threshold conduction switch means between the emitter of the first output transistor and the output of the complementary emitter follower.
The foregoing and further features of the invention are illustrated in the accompanying drawings, wherein like elements are designated by like reference numbers, and in which:
FIG. 1 is a schematic diagram, partially in block form, of a prior art kinescope driver apparatus having complementary emitter follower cathode isolation and AKB current sensing;
FIG. 2 is a schematic diagram, partially in block form, of another embodiment of a prior art kinescope driver apparatus;
FIG. 3 is a schematic diagram, partially in block form, of a kinescope driver apparatus embodying the invention; and
FIGS. 4 and 5 are block diagrams, partially in schematic form, illustrating further features of the invention for effecting load capacitance reduction in conjunction with the apparatus of FIG. 3.